1. Field of the Invention
This invention relates to boiler feedpump turbine systems in general, and more particularly, to an electronic multiple-mode boiler feedpump turbine control system.
2. Description of the Prior Art
Generally, the boiler feedpump turbine systems are considered a part of the overall boiler feedwater supply system and are normally controlled as part of a conventional boiler feedwater control system. The boiler feedpump turbine is usually mechanically connected to and drives a boiler feedwater pump with a common shaft and the amount of water typically pumped by the feedwater pump from a feedwater source to the boiler is usually a function of the rotational speed of the boiler feedpump turbine. Normally, the boiler feedwater pump requirements are coordinated with the speed and load demands of the main turbine system which conducts the steam discharged by the boiler at controlled rates. In many cases, high and low pressure steam sources for the boiler feedpump turbine are supplied from the main steam header and extraction points of the main turbine system. Steam admission valves govern the speed of the boiler feedpump turbine by controlling the rate of steam admission to the boiler feedpump turbines as a function of their position settings. In most boiler feedpump turbine systems, there exists an independent boiler feedpump turbine control system for controlling under closed-loop conditions the speed of the feedpump turbine in an outer loop and the position of the steam admission valves in an inner loop.
One known type of boiler feedpump turbine control system presently offers only two modes of controlling the rotational speed of the feedpump turbine. A first mode permits an operator to control a speed reference set point using increase and decrease pushbuttons on an operator's panel to adjust the output voltage potential of a motor driven potentiometer whereby the voltage potential is representative of the speed reference set point. The rotational speed of the feedpump turbine is controlled in this first mode in a speed range from turning gear speed to a predetermined initial speed suitable for driving the boiler feedpump turbine. In this known system, the rotational speed control of the boiler feedpump turbine is transferred to a boiler control speed signal which is supplied from a conventional feedwater control system when the speed reference set point is initially adjusted equal to a predetermined initial speed value. Control of the rotational speed of the turbine using the boiler control speed signal is considered the second mode of control. After transfer to this second mode of control, the motor driven potentiometer is driven to one side to output a maximum voltage potential. A low select circuit within the control system ensures the continuous selection of the boiler control speed signal thereafter; thus, subsequent to the transfer, the boiler feedpump turbine control system is governed by the boiler control speed signal.
There are certain undesirable features of this type of control system relating to the boiler control signal interface. First of all, there is no automatic detection of an invalid boiler control speed signal. Such an anomaly must presently be detected by a power plant operator usually as a result of observing a disturbance in the operation of the steam supply subsystem of the main boiler/turbine system. In some cases, where the boiler control speed signal fails instantaneously to a state which either demands zero pumping capacity or full pumping capacity, a catastrophic disturbance in the steam supply subsystem may occur of such proportions to affect a trip condition in the main boiler/turbine system thus rendering the system unavailable to produce power. In addition, these present type feedpump control systems offer no limitation to the rate of change in the boiler control speed signal. The rotational speed of the boiler feedpump turbine presently follows any large instantaneous perturbations in the boiler control speed signal and the acceleration of the boiler feedpump turbine is only limited by its inertia and other minor secondary damping factors based on speed. It is understood that turbine acceleration disturbances of this nature will normally occur only occasionally as a result of a contingent electrical noise disturbance in the boiler control speed signal without causing any substantial deleterious effects on the boiler feedpump turbine. However, should a periodic electrical noise disturbance be coupled to the boiler control speed signal, the feedpump turbine may cycle at undesirable accelerations continuously. Due to the periodic nature of the speed changes, there is a possibility that this type of disturbance may go undetected by an operator thereby causing a trip condition to occur which may again render the main boiler/turbine system unavailable as a result of a forced shutdown.
Further, these present type boiler feedpump turbine control systems offer no convenient method for permitting the power plant operator to control the speed of the feedpump turbine across the speed range from 0% to 100% of the rated speed value. Also, no on-line control is presently available to the operator in these type systems to permit overriding the boiler control speed signal supplied by the boiler feedwater control system. In addition, these known systems offer no secondary control systems such as manual control of the valve positions to back up the primary closed-loop speed control system. Such a manual backup valve position controller may allow for a gradual, controlled and planned shutdown of the main boiler/turbine system as a result of certain malfunctions in the boiler feedpump turbine controller which will eliminate in some instances the undesirable forced outages brought on by an instantaneous trip situation. Still further, these present type systems provide only one speed pick-up for the purposes of measuring speed and supplying the speed feedback signal to the closed-loop speed controller. It is apparent that loss of this feedback speed signal due to a failure in the pick-up, for example, will usually cause the turbine to trip as a result of a boiler feedpump turbine overspeed situation.
Presently, it is necessary, in most cases, to operate the boiler feedpump turbines in an unorthodox manner to affect a speed greater than the rated speed of the boiler feedpump turbine for purposes of calibrating and periodic testing of a conventional mechanical overspeed trip weight. In these systems, there are no provisions to permit the operator to conveniently control the turbine speed above the rated turbine rotational speed. In some instances, calibration and testing of the trip weight are done using make-shift modifications to the valve actuator portions of the hydraulic system independently of the boiler feedpump turbine electronic controller. These unorthodox operations consume a great deal of time which could normally be spent more productively.
It appears that improvements to the present type boiler feedpump turbine systems comprising monitoring and detecting anomaly conditions in the boiler feedwater control and reverting to alternative speed and valve position control options are desirable in decreasing the possibility of forced outages of the main boiler/turbine systems in some cases. Operator speed control conveniences and redundancy in essential signals and control subsystems may further enhance the availability and controlability of the boiler feedpump turbine systems. The present invention provides for these and other features to improve the overall control, protection and ultimate availability of the boiler feedpump turbine system.